The following invention relates to an interface or tap circuit for coupling a trunk line to a drop port and more particularly to an interface circuit employing non-linear circuit elements to provide for more efficient and more accurate coupling.
Local area networks employ a coaxial cable bus, sometimes termed a trunk line, to interconnect a number of data processing devices. Different types of such networks employ different standards for network parameters and one such standard is for a token bus network as defined in IEEE standard 802.4 and its identical international counterpart standard ISO 8802-4. This standard defines several technologies that can be used to distribute signals among the communicating drop ports or work stations that are connected to the network bus cable or trunk line. One of the technologies is called "carrierband."
A carrierband network employs a coaxial line as a bus or trunk cable to distribute signals among the plurality of stations, computers, or other data processing devices. Stations are connected to the trunk cable through taps by other coaxial cables frequently called drop cables. A tap may be connected to more than one drop cable.
The tap couples a signal transmitted by a station to the trunk cable such that equal amounts of the signal are sent in each direction on the trunk cable. Conversely a signal from either direction on the trunk cable passing through a tap is split such that a small portion of the signal is transferred to the drop cable and the attached work station while the remainder of the signal continues on the trunk cable.
A conventional trunk cable tap network heretofore employed in the prior art is shown in FIG. 1 in which a plurality of taps, which include connectors coupled in line at various places along a trunk cable, include two drop cables each connected to a separate work station.
The carrierband network standards referred to above define a number of requirements for such taps. One such requirement is that the tap must be passive; that is, it should not contain amplifiers or other powered devices. Also, the portion of the signal taken off the trunk cable and coupled to the drop cable should be -20 dB down from the trunk cable signal. Moreover the impedance of the connections of the tap should be 75 ohms to match the characteristic impedance of the trunk and the drop cable. The trunk cable connections at the tap should have very low reflection, typically less than -35 dB and no more than -20 dB reflection at the drop cable connections.
A schematic diagram of a tap frequently employed in the prior art which meets these requirements is shown in FIG. 2. Each tap includes an in-line transformer and an autotransformer connected to the trunk cable. The in-line transformer includes a resistor connected in series with its secondary coil and the autotransformer connection includes a terminating resistor connected in series between the autotransformer and a drop port.
While the configuration shown in FIG. 2 meets the requirements imposed by the IEEE standard, the resistors that must be used in order to provide impedance matching dissipate most of the signal energy from the drop cable. This makes the connection very inefficient. The inefficiency works both ways. The signal from a drop port is substantially attenuated by the terminating resistors. The result is that the number of data processing devices that can be connected to any particular trunk line is limited.